This invention relates to a radioactive logging technique and more particularly to a prompt fission neutron assaying technique.
When a formation containing a uranium-bearing ore is irradiated with fast neutrons, the ore nuclei react to neutron bombardment by breaking into smaller nuclear fractions which are normally referred to as fission products. This fission is attended by the emission of prompt neutrons immediately upon occurrence of the fission reaction and also by the emission of delayed neutrons by the fission products subsequent to the fission reaction. The prompt fission neutrons are emitted at the time of the fission reaction, while the delayed neutrons are emitted by the fission products for an appreciable length of time following the fission reaction.
The use of fast neutron irradiation for the detection of uranium has also been explored in a paper by Jan A. Czubek, "Pulsed Neutron Method for Uranium Well Logging," GEOPHYSICS, Vol. 37, No. 1, Feb. 1972, pp. 160-173. Czubek examines several phenomena associated with fast neutron irradiation of uranium-bearing formations and concludes that three can be employed to advantage in uranium detection techniques. Those which Czubek proposes for use as uranium content indicators are (1) epithermal neutron intensity resulting from prompt thermal neutron fission of uranium 235, (2) delayed thermal neutron intensity from prompt thermal neutron fission of uranium 235, and (3) delayed thermal neutron intensity from fast neutron fission of uranium 238.
In U.S. Pat. No. 3,686,503 to Givens et al. there is disclosed a borehole logging system for characterizing the uranium content of natural earth formations on the basis of measurements of delayed neutrons resulting from neutron fission of uranium. This patent discloses a subsurface assaying operation which is carried out by locating in a borehole adjacent a formation of interest a logging tool which includes a source of fast neutrons and a thermal neutron detector. The formation is irradiated with repetitive bursts of fast neutrons; and, subsequent to each burst and after dissipation of the original source neutrons, delayed neutrons resulting from neutron fission of uranium are detected. The output from the detector is then recorded in order to obtain a log indicative of the uranium content of the formation.
In co-pending U.S. patent application Ser. No.868,948 filed Jan. 12, 1978, by Wyatt W. Givens and William R. Mills, Jr., entitled LOGGING TECHNIQUE FOR ASSAYING FOR URANIUM IN EARTH FORMATIONS, there is disclosed a borehole logging system employing the prompt fission neutron (PFN) uranium assaying technique. Both epithermal and thermal neutron fluxes resulting from the cyclical irradiation of a formation with bursts of fast neutrons are measured. These neutron fluxes are measured during the time period within each cycle of operation when prompt neutrons resulting from the thermal fission of uranium 235 are expected. The ratio of the epithermal neutron flux measurement to the thermal neutron flux measurement is proportional to the uranium 235 concentration for a fixed borehole diameter and borehole fluid, provided the epithermal and thermal neutron flux detectors have the same spacing with respect to the neutron source and further provided that the neutron fluxes detected by these detectors are measured during the same time period. Anything affecting the thermal neutron flux in the formation also effects the epithermal neutron flux as a measurement of prompt thermal fission neutrons from uranium 235 in the same manner. Therefore, the ratio of the responses of the PFN logging tool to epithermal and thermal neutron fluxes is independent of variations in the neutron generator output and of variations in the formation parameters of porosity, density, and thermal neutron macroscopic absorption cross section. However, the actual epithermal neutron flux to thermal neutron flux ratio measured in the borehole with the PFN logging tool remains dependent upon the variable borehole parameters such as borehole diameter, borehole fluid, and borehole casing.